The centrifugal casting of tubular metal articles in a metal mold which has an inner surface of circular cross section and is rotated about an axis normal to that cross section is very old, iron pressure pipe having been cast in that fashion since the advent of the Delavaud and Sand Spun processes. Using centrifugal casting methods of this general type, it has become common practice to cover the inner or active surface of the metal mold with a lining of refractory material to protect the mold, to prevent the metal being cast from picking up material from the metal mold surface, and to allow the finished casting to be separated from the metal mold. In many prior-art methods, the refractory lining is formed by applying to the metal mold a particulate refractory material bound with a resin binder or an aqueous suspension of, e.g., bentonite, but, though such approaches have gained considerable acceptance, they have the disadvantages that the refractory lining is penetrated unduly by the molten metal being cast, that particules of the refractory material are picked up in the surface of the casting so that finish machining of the casting is difficult and expensive, and that it is difficult to control the thermal conductivity provided by the refractory lining in order to control the type and size of graphite in the metal of the casting. For a number of applications, the method disclosed in my U.S. Pat. No. 4,124,056 has overcome these disadvantages by using a binderless particulate refractory material to establish an initial refractory layer on the active surface of an unvented metal mold, densifying that layer under the action of centrifugal force applied by rotation of the mold, and contouring the densified layer to the precise shape desired for the cast article. That method is based upon the discovery that finely particulate refractory materials such as milled refractory flours, especially zircon flour, can be densified into a lining layer so stable that, e.g., the groove necessary to form an outer flange of the cast article can be cut into the layer with the walls of the groove remaining dimensionally stable after the groove has been formed, the particles of the refractory material after densification and contouring of the layer being packed so tightly together that the lining is at its maximum bulk density and will neither change in shape or be invaded by the molten metal during casting.
However, work with the method described in U.S. Pat. No. 4,124,056 has disclosed two surprising problems when the article to be cast is of such external shape that portions of the refractory lining are required to be radially thick in comparison to the thickness of the molten metal applied to such portions during casting. A first problem arises from the fact that, even through densified to maximum bulk density, the linings of binderless refractory particles contain enough internal voids to trap a significant volume of air and, when the casting temperature is low or the casting metal is thin relative to the refractory lining, trapped air, expanding because of the heat from the cast metal, is forced inwardly through the molten metal not just while that metal is in liquid state but also as the metal begins to solidify, so that undue porosity of the casting tends to occur. The second problem results from the superior insulating properties of, e.g., a lining formed of binderless zircon flour, and the second problem tends not only to accentuate the first but also to make control of graphite size difficult when the metal being cast is iron and specifications require close control of graphite size. Thus, when the article to be cast has a thin-walled portion adjacent, e.g., a thick transverse outwardly projecting flange, that portion of the lining which defines the thin-walled portion of the casting must be markedly thicker than that portion of the lining which defines the periphery of the flange, so that the thermal insulation presented by the lining surrounding the thin-walled portion of the casting is large in comparison with the thermal insulation provided by the lining at the flange. With the metal of the thin-walled portion therefore cooling more slowly, graphite growth in the thin-walled portion of the casting is accentuated. When casting iron blanks for engine cylinder liners, for example, specifications may call for the graphite flakes of the casting to be in a size range of 4-6, but slow heat loss in the thin-walled portion of the casting may result in size 3 graphite. Such large graphite flakes are objectionable because they tend to cause "pull-outs" during machining of the casting. There has accordingly been need for improvement.
Prior-art workers have proposed to control the thermal conductivity of refractory linings in various fashions, but success has been limited to those cases in which a binder was employed in the lining. Thus, it has been proposed to form portions of a refractory lining from different materials, so that one portion would have a different heat transfer capability than other portions, but this has been done only with, e.g., solid rings of high thermal conductivity material for one portion, and the use of solid rings is objectionable. It has also been proposed to use mixtures of different particulate refractory materials, the materials making up the mixture having different thermal conductivities, but this has heretofore not been possible in the case of linings formed without a binder because the particles of such a material tend to classify while the mixture is being applied to the mold surface, such inherent classification resulting in a lining which is not of uniform composition and is therefore unacceptable. Thus, when a mixture comprising a first material of relatively smaller particle size and a second material of large particle size is used, classification occurs according to particle size. Similarly, when particles of two materials of different specific gravity are used in the mixture, classification occurs because of the difference in specific gravity. When a portion of the tubular casting has a wall thickness small in comparison to the thickness of the corresponding portion of the refractory lining, the need for eliminating the air initially trapped in the refractory lining complicates the problem of controlling thermal conductivity of the lining portion when finely divided binderless refractory materials are used, since venting of the lining is difficult because of the tendency for fine particles to clog the air flow passages.